Electrostatics

ELECTROSTATICS Electricity at rest

A Bit of History Ancient Greeks – Observed electric and magnetic phenomena as early as 700 BC Found that amber, when rubbed, became electrified and attracted pieces of straw or feathers Magnetic forces were discovered by observing magnetite attracting iron

A Bit More History William Gilbert – 1600 – Found that electrification was not limited to amber Charles Coulomb – 1785 – Confirmed the inverse square relationship of electrical forces

History Final Hans Oersted – 1820 – Compass needle deflects when placed near an electrical current Michael Faraday – A wire moved near a magnet, an electric current is observed in the wire

Properties of Electric Charges Two types of charges exist – positive and negative – Named by Benjamin Franklin

Like charges repel Opposite charges attract - +

Charges continued The natural order is balanced charges Net charge of zero Unbalanced charges are possible

Question #2 The charge on sphere 2 is three times the charge on sphere 1 Which force diagram is correct? A,B,C,D, or E (none of them)

More Properties of Charge Positive charge carrier is the proton – Protons do not move from one material to another Held in Nucleus Negative charge carrier is the electron (e - ) – An object becomes charged (+ or -) by gaining or losing electrons

More Properties of Charge Electric charge is always conserved – Charge is not created, only exchanged – Charging occurs through the exchange of electrons Lose an electron – Gain a positive charge Gain an electron – Gain a negative charge

Properties of Charge, final The SI unit of charge is the Coulomb (C) Charge is quantized – All charges are a multiple of the fundamental unit of charge, symbolized by (e) – Electrons have a charge of e - e - = x C – Charge of -2 = 2* e - = 2* ( x C) – Protons have a charge of e + e + = x C – Charge of +2 = 2* e + = 2* (1.602 x C)

Conductors Conductors: materials in which the electric charges move freely – Copper, aluminum and silver are good conductors – When a conductor is charged in a small region, the charge readily distributes itself over the entire surface of the material

Insulators Insulators : materials in which electric charges do not move freely – Glass and rubber are examples of insulators – When insulators are charged by rubbing, only the rubbed area becomes charged There is no tendency for the charge to move into other regions of the material semiconductors : characteristics between those of insulators and conductors – Silicon and germanium are examples

Charging… Three ways – Friction Mechanical motion (rubbing) – Conduction (or Contact) Direct contact (no rubbing) – Induction Charge alteration without any contact

Charging by Friction Self-explanatory… (demo)

Charging by Conduction A charged object (the rod) is physically touches the other uncharged, object (the sphere) The same type of charge is CONDUCTED from the rod to the sphere

Charging by Conduction

Charging by Induction Induced charge - NO physical contact between charged & uncharged object OPPOSITE charge is INDUCED

Temporary charge Induction ELECTRICALLY POLARIZED

Permanent charge Induction GROUNDING

Another way to Induce a charge Net Charge of Zero

Question #3 An alpha particle with two positive charges and a less-massive electron with a single negative charge are attracted to each other. The force on the electron is: a)Greater than that on the alpha particle b)Less than that on the alpha particle c)Same as that on the alpha particle d)I haven’t a clue…

Answer #3: (c) Same The force on the electron the same as that on the alpha particle - Newton’s Third Law.

Question #4 An alpha particle with two positive charges and a less-massive electron with a single negative charge are attracted to each other. The particle with the most acceleration is the a)Alpha particle b)Electron c)Neither - they have the same acceleration d)I haven’t a clue…

Answer #4: (b) Electron The particle with the most acceleration is the ELECTRON. Newton’s Second Law (F=ma)

Question #5 An alpha particle with two positive charges and a less- massive electron with a single negative charge are attracted to each other. As the particles get closer to each other, each experiences an increase in: a)force b)speed c)acceleration d)All of these e)None of these

Answer #5: (d) ALL As the particles get closer, the FORCE  and thus the ACCELERATION  and also the SPEED 

Electrical Field Gravitational Field - A force field that exists around any object with mass – Interacts with mass Electric field - A force field that exists around a charged object – Interacts with charges – How do we know it exists? If another charged object enters this electric field, the field exerts a force on the second charged object direction of movement determines charge of the field

Visualizing an Electric Field Michael Faraday developed the concept of drawing Electric Field Lines – Vector quantity – Proximity of field lines indicates field strength – Arrows indicate direction of field Direction indicates the charge – Out of positive – Into negative

Electric Field Lines Point Charge – Field lines radiate equally in all directions Radiate out on positive – Proximity to each other indicates field strength +

Negative Point Charge – Lines point inward Towards the charge -

Electric Field Line Patterns + - Electric dipole - consists of two equal and opposite charges – Add field lines Connected field lines indicates opposite charge – Matching numbers of field lines indicates similar charge values The high density of lines between the charges indicates the strong electric field in this region

Electric Field Lines

Electric Field Line Patterns Two equal but like point charges Zoomed out (far away) – the field would be appear to be one charge Zoom in (close-up) No connections indicate like charges; (repulsion) Low density of field lines between the charges indicates a weak field in region “C”

Electric Field Lines

Electric Field Patterns Unequal and unlike charges Note that two lines leave the +2q charge for each line that terminates on -q

Electric Field Lines, cont.

Electric Field, cont. How do we know they are there? – Interact with charges How do we know what charge they are? – Experimenting (testing) – test charge, placed in the field, will experience a force

Electric Field Testing, cont

+ -

Direction of Electric Field The electric field produced by a negative charge is directed toward the charge – A positive test charge would be attracted to the negative source charge

Direction of Electric Field, cont The electric field produced by a positive charge is directed away from the charge – A positive test charge would be repelled from the positive source charge

Electric Field Mathematically, The electric field is a vector quantity

Question #9 What is the magnitude of the electric field 0.50 meters away from a -3  C point charge? a)1.08 x 10 5 N/C b)-1.08 x 10 5 N/C c)5.4 x 10 4 N/C d)-5.4 x 10 4 N/C e)I don’t have a clue…

Answer #9: (a) 1.08x10 5 N/C What is the magnitude of the electric field 0.50 meters away from a -3  C point charge? a)1.08 x 10 5 N/C b)-1.08 x 10 5 N/C c)5.4 x 10 4 N/C d)-5.4 x 10 4 N/C e)I don’t have a clue…

Question #9 What is the magnitude of the electric field 0.50 meters away from a -3  C point charge? a)1.08 x 10 5 N/C b)-1.08 x 10 5 N/C c)5.4 x 10 4 N/C d)-5.4 x 10 4 N/C e)I don’t have a clue…

Electrostatic Forces If like charges repel and opposites attract… That means there is motion If there is motion there must be a force (F = ma) Newton’s Second Law There must be a way to calculate the electrostatic force!

Coulomb’s Law F = electrostatic force or electrical force k e = electrostatic force constant aka – proportionality constant aka – Coulomb’s Constant = 9.0x10 9 Nm 2 /C 2 q 1 = charge for particle 1 q 2 = charge for particle 2 r = radius (distance between charges)

Question #4 Two charges (+20  C and -10  C) are 3 m apart. What is the magnitude of the force between them? a)0.2 N b)0.6 N c)22.22 N d)2.0 x N F = ? q 1 = +20uC q 2 = -10uC r = 3m k e = 9x109 Nm 2 /C N

Balloon on a Ceiling Which is stronger the force of gravity, or electrical force?

Two point charges are separated by a distance r The like charges produce a repulsive force between them The force on q 1 is equal in magnitude and opposite in direction to the force on q 2 Vector Nature of Electric Forces FORCE OPPOSITE EQUAL

Two point charges are separated by a distance r The unlike charges produce a repulsive force between them The force on q 1 is equal in magnitude and opposite in direction to the force on q 2 Vector Nature of Electric Forces FORCE OPPOSITE EQUAL r

Question #6 If q1 = +20  C and q2 = +10  C and the two charges are 3 meters apart, what is the MAGNITUDE of the force between them? a)0.2 N b)0.6 N c)22.22 N d)2.0 x N e)I don’t have a clue

Answer #6: (a) 0.2 N

Question #7 If q1 = +20  C and q2 = +10  C and the two charges are 3 meters apart, what is the DIRECTION of the force between them? a)Away from each other b)Towards each other c)One chases the other d)Nothing - they don’t move at all e)I don’t have a clue

Answer #7: (a) Away If q1 = +20  C and q2 = +10  C and the two charges are 3 meters apart, what is the DIRECTION of the force between them? Like charges repel

Question #10 What is the electrostatic force acting on a 2 nC charge placed in a 335 N/C electric field? a)0 N b)6.7 x N c)6.7 x N d)6.7 N e)I don’t have a clue…

Answer #10: (c) 6.7 x N What is the electrostatic force acting on a 2 nC charge placed in a 335 N/C electric field?

Electrical Shielding Electrical charges spread over the surface of a conductor in such a way that the net charge INSIDE (at the center) of the conductor is zero

Electrical Potential Energy Energy possessed by a charge by virtue of its location A function of the charge sizes involved and their proximity to one another Electrical PE >> KE

Electric Potential

Electrostatics The End…